The present invention generally relates to machining processes. More specifically, the present invention relates to machining processes requiring selective thermal control and/or lubrication during lathe machining, board cutting, wafer singulation and active electronic component thermal cycling. The present invention may be used as a metalworking and machining fluid for operations such as turning, milling, facing, threading, boring and grooving, and more particularly, to a method and apparatus for performing such metal working operations at high speeds with extended insert life, and more particularly as a direct replacement for flooded coolant/lubricant systems or conventional liquid cryogenic sprays.
Most machining operations are performed by a cutting tool which includes a holder and one or more cutting inserts each having a surface terminating with one or more cutting edges. The tool holder is formed with a socket within which the cutting inserts are clamped in place. The cutting edge of the insert contacts the workpiece to remove material therefrom, typically in the form of chips. A chip comprises a plurality of thin, generally rectangular-shaped sections of material which slide relative to one another along shear planes as they are separated by the insert from the workpiece. This shearing movement of the thin sections of material relative to one another in forming a chip generates a substantial amount of heat, which, when combined with the heat produced by engagement of the cutting edge of the insert with the workpiece, can amount to 1500 degrees F. to 2000 degrees F.
Among the causes of failure of the cutting inserts of tool holders employed in prior art machining operations are abrasion between the cutting insert and workpiece, and a problem known as cratering. Cratering results from the intense heat developed in the formation of the chips and the frictional engagement of the chips with the cutting insert. As the material forming the chip is sheared from the workpiece, it moves along at least a portion of the exposed top surface of the insert. Due to such frictional engagement, and the intense heat generate in the formation of the chip material along the top portion of the insert is removed forming “craters”. If these craters become deep enough, the entire insert is subject to cracking and failure along its cutting edge, and along the sides of the insert, upon contact with the workpiece. Cratering has become a particular problem in recent years due to the development and extensive use of hard alloy steels, high strength plastics and composite materials formed of high tensile strength fibers coated with a rigid matrix material such as epoxy.
Attempts to avoid cratering and wear of the insert due to abrasion with the workpiece have provided only modest increases in tool life and efficiency. One method has been to form inserts of high strength materials such as tungsten carbide. However, while extremely hard, tungsten carbide inserts are brittle and are subject to chipping which results in premature failure. To improve the lubricity of inserts, such materials as hardened or alloyed ceramics have been employed in the fabrication of cutting inserts. Additionally, a variety of low friction coatings have been developed for cutting inserts to reduce the friction between the cutting insert and workpiece. Additionally, attempts have been made to increase tool life by reducing the temperature in the cutting zone, or the area about cutting edge of the insert, the insert-workpiece interface and the area on the workpiece where material is sheared to form chips.
One method of cooling practiced in the prior art is flood cooling which involves the spraying of a low pressure stream of coolant toward the cutting zone. Typically, a nozzle disposed several inches above the cutting tool and workpiece directs a low pressure stream of coolant toward the workpiece, tool holder, cutting insert and on top of the chips being produced. The primary problem with flood cooling is that it is ineffective in actually reaching the cutting area. The underside of the chip which makes contact with the exposed top surface of the cutting insert, the cutting edge of the insert and the area where material is sheared from the workpiece, are not cooled by the low pressure stream of coolant directed from above the tool holder and onto the top surface of the chips. This is because the heat in the cutting zone is so intense that a heat barrier is produced which vaporizes the coolant well before it can flow near the cutting edge of the insert.
Several attempts have been made in the prior art to improve upon the flood cooling technique described above. For example, the discharge orifice of the nozzle carrying the coolant was placed closer to the insert and workpiece, or fabricated as an integral portion of the tool holder, to eject the coolant more directly at the cutting area. In addition to positioning the nozzle nearer to the insert and workpiece, the stream of coolant was ejected at higher pressures than typical flood cooling applications in an effort to break through the heat barrier developed in the cutting area.
Other tool holders for various types of cutting operations have been designed to incorporate coolant delivery passageways which direct the coolant flow across the exposed top surface of the insert toward the cutting edge in contact with the workpiece. In such designs, a separate conduit or nozzle for spraying the coolant toward the cutting area was eliminated making the cutting tool more compact. Finally, machine tools of cutting operations have been designed to incorporate cryogenic coolant delivery through machine tool passageways which direct the coolant flow across the exposed top surface of the insert toward the cutting edge in contact with the workpiece or spray cryogenic fluid such as liquid carbon dioxide and liquid nitrogen, and cryogenic mixtures containing water, directly onto the workpiece to cool and remove chips.
Again, the problem with the aforementioned apparatuses is that coolant in the form of an oil-water or synthetic mixture, at ambient temperature, is directed across the top surface of the insert toward the cutting area without sufficient velocity to pierce the heat barrier surrounding the cutting area. As a result, the coolant fails to reach the boundary layer or interface between the cutting insert and workpiece and/or the area on the workpiece where the chips are being formed before becoming vaporized. Under these circumstances, heat is not dissipated from the cutting area which causes cratering. In addition, this failure to remove heat from the cutting area creates a significant temperature differential between the cutting edge of the insert which remains hot, and the rear portion of the insert cooled by coolant, causing thermal failure of the insert.
Another serious problem in present day machining operations involves the breakage and removal of chips from the area of the cutting insert, tool holder and the chucks which mount the workpiece and tool holder. If chips are formed in continuous lengths, they tend to wrap around the tool holder or chucks which almost always leads to tool failure or at least requires periodic interruption of the machining operation to clear the work area of impacted or bundled chips. This is particularly disadvantageous in flexible manufacturing systems in which the entire machining operation is intended to be completely automated. Flexible manufacturing systems are designed to operate without human assistance and it substantially limits their efficiency if a worker must regularly clear impacted or bundled chips.
Moreover, environmental health and work safety issues are becoming a major concern. It has been estimated that between 700,000 and one million workers are exposed to cutting fluids in the United States. Since cutting fluids are complex in composition, they may be more toxic than their components and may be an irritant or allergenic even if the raw materials are safe. Also, both bacteria and fungi can effectively colonize the cutting fluids and serve as source of microbial toxins. Significant negative effects, in terms of environmental, health, and safety consequences, are associated with use of the cutting fluids.
In an attempt to address some of these issues, the use of oil-water microemulsions has become widespread. The purpose of the emulsion in metal working is to provide maximum cooling with water and at the same time have the oil impart some lubricating properties so that friction between the moving chip and the contact surface of any cutting tool is reduced. However, as a result, the part being machined has a working surface that contains an inorganic contaminant, water, and an organic contaminant, oil. This makes the post-cleaning process much more complicated.
Typically, a solvent cleaning operation is performed in-between or following final processing, which necessitates removal from the manufacturing tool for such operations. For example, a conventional strategy is to remove a machined article from a machining center and using an alcohol to remove the water and an organic solvent to remove the oil. Another conventional post-cleaning operation involves the use of newer organic solvents such as n-propyl bromide (nPB). However, solvents such as nPB are expensive and pose airborne toxicity issues themselves to exposed workers. Moreover, reclamation systems and other associated costs of using organic cleaning solvents such as nPB are prohibitively high.
The use of water-based cleaners and water rinsing is still another method of post-cleaning common in the metalworking industry. Although generally cheaper and safer to use with respect to organic solvents, these agents themselves become polluted with heavy metals and other contaminants and must be treated prior to disposal.
Another deficiency in the prior art is in regard to the use of dry-cold cryogenic sprays to provide selective mechanical force and cooling within a cutting zone of a laser machining operation. Although conventional methods of applying cryogenic sprays to a substrate during machining processes, such as spraying liquid carbon dioxide directly onto the machined substrate to form a cold gas-solid aerosol, may be similarly applied to a laser machining surface, these methods and chemistries suffer from several disadvantages. For example, conventional cryogenic sprays can be used to eliminate laser machining heat and debris, however, because the spray temperature can not be controlled by these conventional processes, significant amounts of atmospheric water vapor is condensed as liquid and solid water in and around the laser cutting zone during the machining operation. Liquid and solid water present on a cutting surface absorb or reflect strongly in ultra-violet and infra-red spectral regions, which interferes with lasing power and beam delivery onto the substrate surface, thus producing cut quality problems. Another limitation is that the spray pressures cannot be controlled effectively to balance laser cutting efficiency with fluid force, temperature and pressure.